In the past, a large number of varying techniques have been developed for the filtration of fluid mixtures. Many of these techniques require passing the fluid mixture through a membrane filter. In general, these membrane filtration techniques may be divided into three basic categories based on filter pore size and filtration pressure. The first of these categories, known as microfiltration, refers to filters having relatively large pore sizes and relatively low operating pressures. The second category, ultrafiltration, refers to filters having intermediate pore sizes and intermediate operating pressures. Finally, the third category, reverse osmosis, refers to filters having extremely small pore sizes and relatively high operating pressures. Predictably, microfiltration techniques are utilized when large solutes, or species, are to be filtered. Ultrafiltration is used when intermediate species are to be processed, and reverse osmosis is utilized when extremely small species are targeted.
Traditionally, membrane filters have functioned by placing a porous membrane perpendicularly across the path of a fluid mixture from which a selected species is to be filtered. The fluid mixture flows through the membrane and the selected species is retained by the membrane. A problem generally associated with traditional filtration techniques is tendency of the filter to accumulate solutes from the fluid mixture that is being filtered. Accumulation of these solutes creates a layer of solutes on the filtration membrane and has a tendency to block, or clog, the pores of the membrane decreasing the flow of the fluid mixture, or flux, through the filtration membrane.
The decrease in flux attributable to the accumulation of the solute layer on the filtration membrane may be partially overcome by increasing the pressure differential, or transmembrane pressure that exists across the filtration membrane. Pressure increases of this type are, however, limited in their effectiveness by the tendency of the filter to become increasingly clogged as the filtration process continues. Eventually, of course, further pressure increases become impractical and the filtration process must be halted and the clogged membrane replaced. This is especially true when biological or other pressure sensitive species are being extracted.
A second problem associated with the accumulation of solutes on the filtration membrane is the tendency for the solute layer to act as a secondary filter. As a result, as the layer of solutes deposited on the filtration membrane increases, passage through the filtration membrane becomes limited to smaller and smaller solutes. The tendency for the solute layer to act as a secondary filter is especially problematic because, unlike the decreased flux attributable to the same layer, it cannot be overcome by increasing the transmembrane pressure.
One solution to the problem of membrane blockage has been the development of tangential-flow filters. Filters of this type employ a membrane which is generally similar to the membrane types employed by traditional filters. In tangential flow filters, however, the membrane is placed tangentially to the flow of the fluid mixture to cause the fluid mixture to flow tangentially over a first side of the membrane. At the same time, a fluid media is placed in contact with a second surface of the membrane. The fluid mixture and the fluid media are maintained under pressures which differ from each other. The resulting pressure differential, or transmembrane pressure, causes fluid within the fluid mixture, and species within the fluid mixture, to traverse the membrane, leaving the fluid mixture and joining the fluid media.
In operation, the tangential-flow of the fluid mixture over the membrane functions to prevent solutes within the fluid mixture from settling on the membrane surface. As a result, the use of tangential-flow filtration has proven to be an effective means of reducing membrane blockage for membrane filters. Not surprisingly, then, a wide variety of differing designs exist for filters of the tangential-flow type. Unfortunately, even when tangential flow filtration is used, there is still some tendency for solutes to accumulate near the filtration membrane. As is the case with dead-end filters, the accumulation of solutes degrades the filtration process, increases the transmembrane pressure and has the twin effects of decreasing the flux of solutes through the membrane and limiting traversal through the membrane to smaller solutes. These problems are particularly acute when the species of interest and other, non-desired species within the fluid mixture have similar molecular weights, as is often the case when mixtures of proteins are to be fractionated. As a result, tangential flow filtration techniques are generally employed only where the species of interest and the non-desired species have molecular weights which differ by a factor of at least ten.
A tangential flow filtration which optimizes the solute concentration at the filtration membrane to increase the ability of the filter to select between similarly sized species is disclosed in U.S. Pat. No. 5,256,294 entitled "Tangential Flow Filtration Process and Apparatus" which issued to van Reis and is assigned to the same assignee as the present invention. For the device disclosed by van Reis, the transmembrane pressure of the filtration process is maintained at a level which is generally less than the transmembrane pressure used in traditional tangential flow filtration systems. More specifically, it has been observed that flux in tangential flow filtration systems increases as a function of transmembrane pressure until the transmembrane pressure reaches a transition point pressure or TPP. Once the transmembrane pressure reaches TPP, flux is relatively independent of further increases in the transmembrane pressure. Traditional tangential flow filtration systems have operated with transmembrane pressure which generally equal or exceed TPP. In comparison, the device disclosed by van Reis maintains the transmembrane pressure of the filtration pressure at a value which is less than TPP. As a result, flux through the device of van Reis may be less than the flux achieved by traditional tangential flow filtration systems. At the same time, however, the ability of the filter to select between similarly sized species is enhanced.
In spite of the development of tangential flow filtration systems and improved tangential flow filtration systems, such as the filtration system disclosed by van Reis, there is a continual need for improved filtration systems. In particular, there exists a need for filtration systems with enhanced abilities to select between similarly sized solutes, such as when mixtures of proteins are to be fractionated. In light of the above, it is an object of the present invention to provide a tangential-flow filtration system for removing solute species from a fluid mixture. Yet another object of the present invention is to provide a tangential-flow filtration system with an enhanced ability to select between similarly size solutes. Yet another object of the present invention is to provide a tangential-flow filtration system which is particularly suitable for the filtration of protein mixtures. Still another object of the present invention is to provide a tangential-flow filtration system which is relatively simple to use, easy to manufacture and comparatively cost effective.